1. Field of the Invention
The present invention relates to a light source device and a light beam scanning optical apparatus, and particularly relates to a light source device and a light beam scanning optical apparatus which are to be used as image writing means in a laser printer, a digital copying machine or the like.
2. Description of Prior Art
There has been known a type of light beam scanning optical apparatus which simultaneously converges a plurality of light beams on different locations on a scanning surface and thereby simultaneously writes a plurality of lines in one scanning operation.
With regard to the switching of the image densities in such a light beam scanning optical apparatus, there have been proposed a method of switching over the intervals between light beams by the movement of a prism or the like disposed ahead of a light beam coupling device along the optical path, a method of switching over the intervals between light beams by the rotation of the light source, a method of increasing or decreasing the number of active light emitting points in accordance with an image density, and the like.
The conventional light beam scanning optical apparatus, however, requires the prism in movable parts thereof to be moved with an accuracy on the order of 0.1 .mu.m or requires the light source to be rotated at the same accuracy, which necessitates a great difficulty in position control. When the method wherein the light source is rotated or the method wherein the number of active light emitting points is adjusted according to an image density is adopted, a laser diode array having a plurality of light emitting points is employed. In the apparatus, narrowing the intervals between the light emitting points causes thermal crosstalk between the light emitting points and thus may result in variations in quantity of light among light beams and degradation in image. On the other hand, broadening the intervals between the light emitting points may result in too large intervals between light beams on the scanning surface; in this case, narrowing the intervals on the scanning surface by an appropriate setting of the magnification of the optical system involves an extremely small focal depth or an extremely large loss in quantity of light caused by apertures.
As a light source device for such a light beam scanning optical apparatus which simultaneously writes a plurality of lines in one scanning operation, light emitting points arranged in a line have been conventionally employed. In the case that the conventional light source device has three or more light emitting points, however, the distances from the light emitting points to the optical axis differ and the positions of the light emitting points are not equivalent optically relative to a convergent lens which shapes the light beams emitted from the light emitting points into generally parallel or convergent bundles of rays. As a result, variations occur in convergence of the light beams, e.g., in that the aberrations of the light beams emitted from the light emitting points disposed on both sides (the light emitting points positioned remotely from the optical axis) are greater than the aberration of the light beam emitted from the light emitting point disposed in the center (the light emitting point generally on the optical axis); the uniformity of image is thus impaired.
In addition, three or more light emitting points have nonequivalent positional relations to each other; accordingly, an increase in temperature of the light emitting point disposed in the center is greater than that of the light emitting points disposed on both sides, and thus variations occur in quantity of light among the light beams.
Further, as an example of a light beam scanning optical apparatus which simultaneously writes a plurality of lines in one scanning operation, in Patent Publication No. 6-48846, for example, a technique has been proposed in which the setting of the intervals between light emitting points of a light source is made flexible by performing interlaced scanning. Interlaced scanning means scanning previously the lines between the lines to be scanned afterward, as shown in FIG. 48, as distinct from scanning lines sequentially from the front extremity of an image with a plurality of light beam spots 81a to 81d as shown in FIG. 47.
The interlaced scanning will be specifically described below in detail, referring to FIG. 48: The intervals between a plurality of light beam spots 82a to 82d with respect to the direction of sub scanning on a photosensitive member are set uniformly at three times of an interval P which is required to form an image of a desired density. In a first operation of scanning, the light emitting points producing the spots 82a and 82b on the photosensitive member are not lighted, while only the light emitting points producing the spots 82c and 82d are lighted concurrently. In a second operation of scanning, only the spot 82a is not lighted, while the remaining spots 82b to 82d are lighted concurrently. In a third and later operations of scanning, all the spots 82a to 82d are lighted concurrently. In the first and second operations, some spots are not lighted for preventing a failure in scanning at the front extremity portion of an image. Thus the plurality of light beam spots 82a to 82d form the image (an electrostatic latent image) on the scanning surface, while the spots 82c and 82d write lines ahead of lines to be written by the spots 82a, 82b in later scanning.
Such a conventional light beam scanning optical apparatus, however, has a constraint of requiring uniform intervals between the beam spots 82a to 82d on the surface to be scanned, which imposes restrictions on the individual shapes and arrangement of a deflecting device, scanning optical elements and the like. In the case that a polygon mirror is employed as the deflecting device, for example, an optical system which converges light beams on the deflection surface of the polygon mirror in the direction of sub scanning may be provided to correct errors in perpendicularity of the deflection surfaces of the polygon mirror; however, uniform intervals between the beam spots on the scanning surface, as typical in the conventional light beam scanning optical apparatus, make it difficult to optimize the magnification of the optical system. Besides, uniform intervals between the beam spots with respect to the direction of main scanning on the scanning surface similarly impose restrictions on the individual shapes and arrangement of a deflecting device and scanning optical elements.